The diva is back! Though she didn't mentor any contestants during the fourth season of "The Voice," Christina Aguilera made a grand return for the NBC hit's Tuesday night finale. Showing off her estimated 20-pound weight loss in a black pencil skirt, a sheer metallic top and a hot pink bra, the four-time Grammy winner performed her No. 1 hit "Feel This Moment" with Pitbull.

"How cool was that seeing Christina tonight?" Blake Shelton asked reporters backstage. "She looks amazing, by the way." Aguilera and Cee Lo Green will join Shelton and Adam Levine for Season 5 after Shakira and Usher stepped in for Season 4.

"Cee Lo and Christina are the two that Adam and I started this thing with and there's a camaraderie there that helped create this thing," said Shelton, who celebrated his 37th birthday that night. "It will be right back where it was before. You know, there's a friendship there -- I still talk to them both all the time, and I'm sure Adam does, too."

In her off season, Aguilera, 32, has been working hard to get her body back in shape. In early May, with a slew of appearances lined up for her new single, the pop star made a drastic change: She signed up for the Fresh Diet premium meal delivery plan. "She did it all on her own," an insider revealed in the June 10 issue of Us Weekly. "She goes online and picks out her own food."

Aguilera didn't just make over her diet. The mom to Max, 5 (with ex-husband Jordan Bratman), has recommitted herself to exercise, reteaming with fitness pro Tee Sorge, who helped her shimmy down for 2010's "Burlesque." In addition to weight training with Sorge, Aguilera has been practicing yoga regularly. "It's more of a mind-body thing for her," said a source. "She believes in exercise that she enjoys, doing strength work as opposed to intense cardio."

The multiplatinum pop star is embracing her slimmer shape, according to stylist Simone Harouche. "She's really into dresses that highlight her decolletage and her waist," she told Us. "She's showing off her amazing figure."